UPDATE
I managed to get my hands on a miniature pcb relay. The fans are now powered by the PSU and duet3 24V always-on activates the relay coil.
It is working as intended now, M999 does not turn off the board.
I am guessing that power drawn from the 24V always-on should not exceed 100mA, as for now I have around 40mA connected.
Thank you everyone for your inputs 